Potent attenuation of context fear by extinction training contiguous with acquisition.

Studies on the behavioral mechanisms underlying contextual fear conditioning (CFC) have demonstrated the importance of preshock context exposure in the formation of aversive context memories. However, there has been comparatively little investigation of the effects of context exposure immediately after the shock. Some models predict that nonreinforced context exposure at the end of the acquisition session will strongly influence the strength of conditioning and/or recruit distinct neural mechanisms relative to extinction after acquisition. Here we investigate the effects of manipulating postshock context exposure on CFC in mice. Prolonging the period of context exposure immediately following the shock caused a significant and durable reduction in conditioned fear. This immediate postshock context exposure was more effective at attenuating conditioned fear than was an equivalent amount of context exposure a day or more after acquisition. The results suggest that nonreinforced exposure to the context influences conditioned fear through distinct mechanisms depending on whether it occurs during acquisition or after it. The superiority of immediate postshock context exposure was specific to single-shock CFC; in two-shock CFC, immediate and delayed postshock context exposure had similar effects. Consistent with previous reports, we hypothesize that the effectiveness of extinction is modulated by emotional state, and procedures engendering higher postshock freezing (such as two-shock CFC) are associated with weaker immediate extinction.